There is a difference between targets and collateral damage. As heinous as McVeigh's crime, I think it is pretty widely known and accepted that his target was the government and he frankly didn't care who was in the building.
Let's choose another example, since the Mass Murderer of Men, Women and Children named Tim McVeigh seems to be too charged a subject.
I remember what it was like during the Atlanta Child Killings. Young people were specifically targeted. Children. The killer was an American. Some of his victims were adults as well. Jeffrey Dahmer, I believe killed minor boys among his victims. John Wayne Gacy? Can't remember for sure. Look at this young child from Utah and the one from California. People do in fact target children and they are in fact Americans.
To say that no American sniper (whatever that means) would target children because American killers have social taboos against killing children is clearly preposterous. It doesn't add up. A cold blooded killer like this sniper(s) has zero respect for human life. I don't think it makes one whit of difference to him whether his victims are black, white or orange, male or female, grownup or child.